The invention relates to communications systems and methods, and more particularly to a data communications system and method in which channel impairments, including a digital loss in transmitted signals imposed by a switch in a central office, are treated.
It is well known that a public switched telephone network (PSTN) comprising Ti facilities can form a basis for a virtual digital network providing 64 kb/s channels. For example, by synchronizing a pulse code modulation (PCM) modem to an 8 kHz sampling rate provided in a central office and using 8-bit PCM words for data transmission, the modem can theoretically achieve a data rate up to 64 kb/s.
However, in practice, due to power constraints and such channel impairments as echo and intersymbol interference, the highest data rate achievable by the PCM modem is about 56 kb/s. This rate may be further reduced as the central office periodically xe2x80x9crobsxe2x80x9d the least significant bit (LSB) of the PCM words and substitutes it with a signaling bit. As is well known, the robbed bit signaling is necessary for indicating call statuses to effect call administration in the PSTN.
To reduce echo interference in traditional voice communications, especially far echo interference due to a long-distance feedback of a voice signal through the PSTN, the level of the voice signal from the PSTN is attenuated in a central office switch before it is passed onto an analog loop connected to telephone equipment. Such attenuation by the central office switch is known as a xe2x80x9cdigital loss.xe2x80x9d
While the above robbed bit substitution does not cause significant distortion in voice communications, it causes significant degradation in data communications because of the loss of transmitted bits occasioned thereby. Similarly, while the above digital loss helps reduce the far echo interference in voice communications, it causes the levels of transmitted signals representing data to be attenuated, resulting in erroneous data recovery in data communications if the digital loss is not taken into account in the PCM modem. Although the digital loss is built into each central office switch and the underlying attenuation factor is invariant as far as the switch is concerned, this factor may vary from one switch to another depending on its type and manufacturer. As a result, a PCM modem which is pre-adjusted during manufacture thereof to allow for the digital loss by a particular switch may not function properly when connected to a different switch in the field.
Accordingly, there exists a need for effectively treating, apart from the robbed bit signaling, the digital loss imposed by a central office switch in data communications, even without knowing its actual attenuation factor a priori.
In accordance with the invention, a signal level conversion table is created using a xe2x80x9clevel learningxe2x80x9d process, which forms part of training of a PCM modem before its operation. This table contains each allowable transmitted signal which is affected by the digital loss imposed by the central office switch, and the received signal level corresponding thereto. During operation of the PCM modem, by looking up this conversion table, the most likely transmitted signal corresponding to a received signal level is effectively determined, thereby recovering the underlying transmitted data. Advantageously, with the invention, the actual attenuation factor applied by the switch to a transmitted signal does not need to be known a priori to properly treat the resulting digital loss.
Specifically, in the level learning process in accordance with the invention, multiple sequences of training signals are transmitted one by one to the modem. Each sequence of training signals is associated with a signal level of a different allowable transmitted signal. Selected training signals in each sequence have a signal level the same as that associated with the sequence. The levels of signals received by the modem corresponding to a subset of the transmitted training signals are xe2x80x9clearnedxe2x80x9d and used to create the aforementioned table.